Process of mixing



Nov. 25,1969 l w. 5mm sm 3,480,261

PROCESS OF MIXING Filed- April 1o, 1968 5 sheets-gr'lt 1 l F/G.

@0m/ey Aff/xm I v0 @a I 'i Nmr.V 25, 11969 Filed April `l0 w. ElRlcH ErAL 3,480,261

PROCESS OF MIXING v 5 Sheets-Sheet 2 kan/mw Nov. 25, 1969 v w- Elmer-lETAL 3,480,251

PROCESS oF MIXING Filed April 1o, 1968 5 sheets-sheet s Nov.\25, 1969 w,ElmcH ETAL PRocEssnFA vMIXING 5 Sheets-Sheet 4.

Filed April 10 1968 m, wf

Nov. 25, 1969 w. r-:nRlcH ETAL 3,480,26

PROCESSO? MIXING Fired April 1o, 1958 A 5 sheets-sheet s United StatesPatent O nu. cl. Bzsc/2o, 7/02 U.S. Cl. 259-148 8 Claims ABSTRACT OF THEDISCLOSURE The disclosure relates to a process for the intensive mixingof the ingredients in the preparation of construction materials such asconcrete, calcium silicate brick or the like and includes the steps ofvarying the speed of a mixing element and/or adding of water so as tomaintain the energy input substantially constant at the rated capacityof the power operated drive. In one example the mixer is initiallyoperated at a speed to produce the rated capacity and water is graduallyadded simultaneously with a gradual increase in speed to maintainoperation at rated load. In another example all the remaining waterrequired is added at one time and the speed of operation of the mixingelement is correspondingly increased.

least one of which can be operated selectively at different speeds inseveral stages.

By such a method for intensive preparation of construction materialsusing a binder of lime or cement ground to the nest degree ofsubdivision, a high strength, particularly a high early settingstrength, can be achieved and the amount of binder can be reduced incomparison to the conventional methods of preparation. Such a method ofpreparation also permits the largest possible amounts of energy to beintroduced into the materials being mixed in a relatively short periodof time, which results in a large output per unit weight ofthe materialsbeing mixed. However, the driving eiiiciency for the mixers used for thepreparation of construction materials cannot simply be increased atwill. For instance, the energy absorption of the material be-ing mixedis very strongly influenced by its consistency. In the region of arelatively low consistency (relatively high water/cement factor), thespecific energy absorption of the mixture is, for example, so low thatthe desired advantage cannot be achieved. A further consideration ofthis k-ind of preparation is that a slight variation of the mixingprocedure must be made for different concrete formulations. The bindercontent also influences the energy absorption of the mixture so that,for example, mixtures having a relatively low cement content produce analmost constant energy absorption whereas mixtures high in cement withan increasing proportion of water produce a sharp decrease in energyabsorption.

The object of the invention is to provide a process for the preparationof construction materials in which a relatively large amount of energycan be introduced into a mixture in a short period of time and thebinder content can be made more useful; and one in which the drivesystem for the mixer used in the preparation can be driven at itsnominal or rated load dur-ing most of the entire mixing procedure sothat a greater amount of energy 3,480,261 Patented Nov. 25, 1969 ice isemployed than when a batch is prepared by conventional methods. Whereas,it has heretofore been assumed that when mixing materials, such asconcrete, a uniform and homogeneous distribution of the components wasdecisive in achieving the highest possible value in the nal product, thepresent invention resides in the `discovery that by the furtherintroduction of energy after the achievement of a completely homogenousmixture, the quality of the material, particularly its compressivestrength, can be considerably increased, or if the same compressivestrength is to be produced, a saving in binder of up to twenty percentcan be achieved.

In accordance with the present invention the intensive preparation ofmaterials such as concrete, with strength and/or saving in binder, isaccomplished by mixing a binder and aggregate with quantity of thinner,such as water, and in such a manner that the mixer operates near itsrated load and the mixing continued after the achievement of ahomogenous mixture until further mix-ing work produces no furtherincrease in the quality of the construction produced from the mixedproduct. The achievement of this ultimate condition is determined by aseries of tests involving a variation in Water/cement ratio, theduration of mixing, and the speed of the rotating element or elements ofthe mixing apparatus.

If, in a given case, as is particularly true in the case of concretewith a low water/cement factor, the added water required as determinedby such tests, gives the consistency desired for further working, thenthe mixing procedure can be carried out in a single stage. In manycases, however, further addition of water after conclusion of theintensive mixing will be desirable or necessary.

According to one embodiment of the invention, cement and aggregatematerials are initially combined with a fractional portion of the totalamount of water required, and the mixing procedure is carried out duringa period of operation of the mixing apparatus at a speed at which themaximum output of the drive for the mixer is introduced into themixture. The remaining water is then introduced with a simultaneousincrease in the speed of rotation of the mixing apparatus so that themixer continues to operate at its nominal or rated load and the energyinput into the mixed materials remains at the same value. By this kindof two-step process at different speeds, it is possible to introduce alarge amount of energy into the mixed materials in a relatively shorttime so that an intensive reduction in particle size takes place whilethe mixing apparatus is simultaneously driven at its rated load. It is-important that gel forming on the particles of cement be rubbed olf, sothat gel formation and hydration proceeds right to the core of theparticle. The aggregate (stone and/or sand) functions in the process asa grinding medium and produces silica powder by abrasion. A furtheradvantage is that the mixer can be driven during the preparation of thematerials so that the drive motor runs at its nominal or rated capacityduring the entire course of the work. The fractional amount of waterused in the introductory step of the process, as well as the remainingWater added in the second step carried out with an increased speed ofthe mixer, can be adapted to a variety of different concretecompositions so that the energy input into the mixture during the entirepreparation of concrete remains at substantially the same value.

According to a further feature of the invention, the mixing procedure isbegun near the maximum energy input which the drive can safely utilize,which is a function of the water/ cement ratio on the decreasing portionof the curve, and the addition of water follows continuously with acorresponding continuous increase in the speed of the rotating elementsof the mixer to maintain the energy input substantially constant.

Advantageously, according to a further feature of the invention, themaxing procedure is begun at an energy level approaching the maximumenergy input to the mixer as a function of the water/cement ratio, andthe remaining water is added in one portion with a simultaneous increasein the speed of the mixer in a single step. In this manner, a steepfalling ot of the curve of energy input versus water/cement ratio isavoided on addition of the remaining water. In this procedure, thenumber of revolutions is adjusted so that the drive motor of the mixeris operated at about its rated load in both mixing phases.

It has further been shown that the throughput of the above describedpreparation method can be increased by heating the concrete in themixture. By heating, the strength of the early set concrete can be soincreased that molds, frames and forms may be quickly removed fromconcrete bodies of large dimensions. For example, molds can be usedthree times within a twenty-four hour period. Such heating preferablytakes place during mixing by the introduction of saturated steam at apressure of 2.5-3 kg./cm.2, in a conventional manner. The temperature ofthe concrete should not exceed 60 C., however. For this embodiment,Portland blast-furnace cement is particularly suitable.

The mixing procedure may be carried out in a countercurrent mixer with amain mixing system continuously rotating at uniform speed and a variablespeed mixing element or agitator. When such countercurrent mixers areemployed, the high speed mixing element strongly promotes the intensivepreparation of mixed materials. In principle, it is possible toconstruct a mixer so that the rotative speed of the machine isadjustable over a wide limit, in which case the same machine can takeover the function both of a mixing shovel and of a high speed mixingelement. Mixing shovels are principally necessary for emptying themachine. By the use of a barrier wall it it possible to avoid the use ofywide-area mixing shovels, so that all rotating systems can operate witha relatively high rate of speed and thereby noticeably increasing theoverall effect.

A better understanding of the present invention will be had from theaccompanying drawings in which FIG. 1 is a plot of several curvesshowing the energy requirement of a rotating type mixing element of acountercurrent mixer at different speeds of rotation of the element andfor mixtures of diiTerent consistencies;

FIG. 2 is a further plot of curves showing the continual increase in thespeed of rotation of the mixing element with a continuous addition ofwater during the second process step of the present invention;

FIG. 3 is a schematic view of an apparatus suitable for carrying out theprocess of the invention;

FIG. 4 is a schematic plan view of the apparatus; and

FIG. 5 shows a drive mechanism for the mixing apparatus of FIGS. 3 and4.

The curves of FIG. 1 were determined from using a countercurrent mixerhaving a rotary mixing element and loaded with a mixture comprising 750liters of aggregate, 175 kg. of cement, and 140 liters of water. Therotating element had the form of an H. Lower curve I was determined atan agitator revolution of 200` r.p.m., and upper curve II was determinedat an agitator revolution of 400 r.p.m. Both curves clearly show thevery different total energy input of the drive motor of the mixer fordifferent numbers of revolutions and different consistencies of themixture.

Curves I and II of FIG. 2 were plotted from data obtained from the samemachine when filled with the same material as in FIG. 1. Upon reaching anominal or rated load for the machine motor of about 40 kw. at point 1,the rotative speed of the [cyclone type] mixing element was continuouslyraised from 200 r.p.m. to 400 r.p.m. at point 2 with continuous additionof water until a desired water/cement ratio of 0.75 was reached. It canbe seen from FIG. 2 that according to the process of the invention themaximum energy input to the machine can be held at a substantiallyconstant value, here shown as 40 kw., despite an increase in therotative speed to 400 r.p.m. In contrast, if the same mixture ofaggregate, cement, and water at a water/cement ratio of 0.75 wasintroduced initially into the machine and the rotating mixing elementwas run at a uniform speed of, for example, 400 r.p.m., during theentire mixing procedure, no intensive utilization of the mixing elementin the preparation of the mixture or of the binder particles would beachieved because the mixture would not have the necessary consistencyfor energy intake. By operating in two steps, and during the irst steponly so much water is introduced as is necessary for wetting so that thewater/ cement ratio is in the region of about 0.3 to 0.4, a tinereduction of the binder is brought about from the very beginning andthen is further increased in the second step of the process uponincreasing the number of revolutions of the rotating mixing element andupon addition of the remaining quantity of water until the water/ cementratio is about 0.65, for example. Depending on the kind of mixture beingworked, the first step of the intensive preparation requires only a veryshort period of time (for example 40-70 seconds), whereupon the additionof the remaining water follows and the number of revolutions of therotating mixing element is increased. The latter is raised from time totime as the consistency of the mixture softens upon the addition ofwater, so that the drive motor is kept near its nominal or rated load.The mixture can be discharged after as little as an additional '20-40seconds of operation.

The times given, by way of example, correspond to those usuallyencountered in practice. Nevertheless, it has been shown that longermixing times are unusually economical if the highest possible savings incement consumption are desired. Mixtures having a high proportion ofcement are expensive merely because of their composition of rawingredients. In the preparation of a concrete having 400 kg. of cementper cubic meter, for example, more grinding work must be expended thanfor a concrete having only 200 kg. of cement per cubic meter. Accordingto the present process, the cost of the increased energy for additionalmixing amounts on the average to only 3-5 percent of the resultingsaving in cement, so that the increased expenditure of mechanical energyproves extraordinarily economical. The maintenance of the energy demandof the drive motor at a constant value is accomplished for differentconcrete formulations on the one hand by graduated water additions and,on the other hand, by varying of the speed of rotation of the mixingelements. It has been found that the energy demand of the drive motorfor a rotating mixing element increases almost proportionally with theincrease in the number of revolutions thereof, and that this ratio isVariable within certain limits depending on the consistency of thematerial being mixed. Concrete mixtures having a low cement content, forexample, show an almost constant, relatively high, specific energydemand over the entire range of consistences conventional in concretetechnology. Concrete mixtures having a high cement content, in contrast,have a steeply decreasing specic energy demand with increasingwater/concrete ratio. With the addition of the remaining quantity ofwater required in one portion of the second process step together with asimultaneous discontinuous increase in the number of revolutions of therotating mixing element, care must only be taken that the rated load ofthe drive motor for the mixing element is not exceeded. The advantagesachieved in this embodiment correspond essentially with those ofcontinual water addition and continual increase of the number ofrevolutions of the drive motor. In case of large variations in themoisture content of the concrete aggregate, the process of the inventioncan also be used for maintaining a certain constant concreteconsistency, particularly when the consistency lies in the region of thesteeply decreasing Ibranch of the energy requirement curve. In thiscase, the addition of water is stopped as soon as the energy input ofthe mixer has reached a value corresponding with the desired linalconsistency. Subsequently, the rotative speed of the mixing element isadjusted to the maximum permissible load and the intensive preparationis completed. For the preparation of very soft mixtures, a firstquantity of water can be added depending on machine load, that is, theaddition of water is discontinued when a value in the neighborhood ofthe energy maximum is reached. At the conclusion of the intensivemixing, any residual portions of water still lacking can be quicklyintroduced with a water meter and the mixing continued, or the step ofadding the remaining water may be eliminated if the smaller drive demandcorresponding with the desired region of consistency is reached.

Variable speed electric or hydraulic motors can be employed to drive themixing element. Regulation of the rotative speed can be carried out bymeans of automatic control switches either automatically or by hand.

Example I An aggregate having a particle size of from 0-30 mm. and asuitable amount of cement, for example in a ratio of 3400 kg. aggregateto 500 kg. cement, is supplied to a countercurrent mixer having a mixingapparatus and a cyclone type mixing element. Water is then added asquickly as possible in an amount such that the drive motor is loaded toits full rated load. In practice, a wattmeter will initially indicatethat the energy input exceeds the rated load of the motor during thecourse of water addition. So much additional water then must be addedthat the watt meter returns to the rated load for the motor. For thiscomposition, intensive mixing takes about a minute. However, theduration of the intensive mixing is dependent on the amount of cementemployed and for larger quantities of cement, a longer intensive mixingis necessary in order to achieve line reduction of the numerousparticles. Prior test runs are used to determine which specific powerconsumption and in which time the optimum cement yield can be obtained,that is, the most satisfactory relationship between early strength,iinal strength and ease of workability for a particular concretecomposition. After conclusion of this intensive mixing, the remainingamount of water is added in order to bring the concrete to the desiredconsistency, and then the mixture is removed from the mixer.

Example 2 Cement, sand, and gravel are introduced as dry components intoa countercurrent mixture as above and the total quantity of waterdesired for the nal consistency is added. The variable speed mixerelement or elements are then set at such an r.p.m. rate that the ratedload of the drive motors is reached. Mixing is continued at rated loadby varying the speed of rotation of the rotary mixing element orelements until the optimum binder utilization has been achieved, asdetermined by prior exploratory tests. This in in distinction to themixing techniques heretofore used in the art where the mixing machinealways operates at the same rotative speed independent of theconsistency and of the kind of concrete being prepared. Nevertheless,even in the preparation of concrete of a relatively soft consistency,the amount of energy necessary for an intensive preparation are notused.

For an economic intensive preparation or mixing, it is of decisivesignilicance that the machines used operate at full load capacity duringthe entire preparation process, even when the material being handled hassharply varying properties. Overloading and partial loading should notresult because of the transitions between regions of differentconsistency required according to the present process. For this reason,a regulation of the speed of rotation depending on load is necessary.For example,

hydraulic drives can be used. This kind of drive is rela.

tively expensive, since a pump and an hydraulic motor controllable overa wide r.p.m. range, together with the accompanying expensive oil supplyregulation, are necessary in addition to an electric motor. Remotelycontrolled variable speed regulators also can be employed in principle.For the loads in question these are not only very expensive, but alsocause difficulties with installation because of their dimensions. Directcurrent special motors also can be adjusted over a wide r.p.i. range.They also are relatively expensive in contrast wit-h alternative motorsand, in addition to the direct current motor, a direct current generatormust be used. Pole changing motors have a control region which is toosmall for intensive preparation or mixing of the invention in general,or, in other words, for good utilization of the apparatus which requiresa control region of about 1:10. Variable alternating current motors dodevelop good torque characteristics as soon as the desired number ofrevolutions is reached. On the other hand, these motors in theirstandard form have the disadvantage of not being able to accelerateunder a heavy load from a low to a high rotative speed.

Proceeding from these considerations, a relatively simple driveapparatus, shown in FIGS. 3-5, has been developed to meet the demands ofan intensive preparation according to the invention. The apparatusconsists essentially of a mixing bowl 1, which from the point of view ofits construction and drive corresponds with the disc of a countercurrentmixer, and having a closure disc 2 for a discharge opening, a high speedmixing element 3 which operates to produce mixing, and a movableemptying blade 4 positioned as shown in full lines when the dischargeopening is open, and, during the mixing procedure, being either in theposition shown in dotted lines in FIG. 3 or lifted above the material=being mixed, or in the concentric position shown in dotted lines inFIG. 4. As shown by the arrows in FIG. 4, the mixing bowl 1 may berotated in a clockwise direction by conventional mechanism not shown inthe drawings and the high speed mixing element 3 may be rotated in acounter-clockwise direction by mechanism described hereafter.Positioning rod 5 is either movable vertically (FIG. 3) or rotatablewithin a limited angle (FIG. 4).

The drive for the drive shaft 6 of the rotating .mixing element 3 isshown in FIG. 5. An electric motor 7, operable in at least two speeds,drives a V-grooved pulley 8 or a gear wheel at a corresponding ratio,for example 4:1 or 5:1. On each of its upper and lower faces, pulley 8carries clutch discs '9 and 10, each of which is provided with a clutchfacing. The opposing clutch halves each have a pinion 11 and 12 linkedto shaft 6 of the mixing element in a ratio of 1:1 or 1:4 (or 1:5). Theclutch discs are positioned on a stationary shaft 13 which is, however,vertically moveable. If the machine is to run at the lowest speed, thedrive motor is started and brought up to a high speed of, for example,1500 r.p.m. Then shaft 13 carrying the clutch discs is lifted and clutch10 is brought into engagement. In analogous fashion, the upper couplinghalf 9 is activated. The shifting of the clutch can take place manually,or by electric motor, through a small worm and wheel drive with limitswitching. Continual contact pressure and maintenance of the medianposition is achieved by means of a spring introduced into the verticallymoveable arrangement. Starting and switching of the drive motor followsfrom time to time from the idling position with the clutch in anintermediate position. All gears remain in mesh. It is possible tochange the direction of rotation if desired.

The emptying blade 4 illustrated in the drawings can be omitted inmixers having bowls which can be tipped for emptying.

The curves of FIGS. l and 2 and the working examples pertain to mixturescontaining cement. However, the invention is also applicable tomaterials containing lime as a binder, for example calcium silicatebrick (sandlime brick) mixtures. In the latter, the avoidance of binderaggregates and the promotion of sand abrasion are critical because thepresence of suiiiciently tine quartz powder s necessary for thedevelopment of the binding strength of the lime.

According to the process of the invention, a higher initial strength isobtained as the final result but, in many cases, an increase in nalstrength and an improvement in workability are also achieved. Also, inmany cases the smoothing of the particle surfaces brought about byintensive mixing is an advantage, because rough particle surfaces leadto an increase in consumption in water and cement without any increasein compressive strength.

We claim:

1. A process for the intensive mixing of the ingredients of aconstruction material with the aid of a mixing apparatus having at leastone variable speed mixing element and a power operated drive for themixing element, which comprises the steps of supplying an aggregate,binder and thinner to the apparatus, adding thinner to the apparatus forvarying the consistency of the mass, rotating the mixing element atdifferent speeds in the materials supplied to the apparatus to mix thematerials at the rated capacity of the drive, and varying at least oneof the steps consisting of varying the speed of the mixing elements andadding thinner to the apparatus so as to maintain the energy inputsubstantially constant at the rated capacity of the power operated drivefor at least the time required to produce a homogeneous mixture, therebyto produce a. more intensive utilization of the mixing apparatus.

2. A process in accordance with claim 1 in which the binder is cement,the thinner is water and the mixing is continued until no increase inthe quality of mixed product can be obtained.

3. A process in accordance with claim 2 in which the Water and binderratio initially supplied requires an energy input near the maximumcapacity of the power operated drive.

4. A process in accordance with claim 2 in which only a portion of thetotal amount of water required is initially supplied to maintain theenergy input at the rated capacity of the drive, subsequently adding theadditional water required, and simultaneously increasing the rotationalspeed of the mixing element so as to maintain substantially the sameenergy input to the mixing element.

5. A process in accordance with claim 4 in which all of the additionalwater required is added at one time and the corresponding increase inspeed of rotation of the mixing element is produced in one step.

6. A process in accordance with claim 4 in which the additional Water isgradually added and the speed of rotation of the mixing element isgradually increased.

7. A process in accordance with claim 2 in which the materials areheated to a temperature not exceeding 60 by the addition of saturatedsteam as they are being intensively mixed.

8. A process in accordance with claim 1 in Which the mixer has a mixingelement rotatable in one direction to act on the material during mixing,and a Variable speed rotating element rotatable in a countercurrentdirection to said rst mentioned element.

References Cited UNITED STATES PATENTS 3,062,514 11/1962 Waimer.3,081,983 3/1963 Thibodeaux. 3,160,400 12/1964 Harrison. 3,326,5356/1967 Clerex 259-147 ROBERT W. JENKINS, Primary Examiner U.S. Cl. X.R.259-85, 177

